1. Field of the Invention
The present invention relates in general to a system and method for digitally embedding information and, in particular, to a system and method for embedding digital information into digital or analog media and later detecting the embedded information using spread spectrum modulation.
2. Related Art
Embedding digital information into a digital or analog media has many important applications including counterfeiting reduction. For example, copyright information may be hidden within a digital signal (such as an image, video or audio), distributed with the digital signal and then later detected to ensure authenticity. The technique of embedding information within a work in a way that is not immediately perceptible and is hard to reproduce (also called xe2x80x9cwatermarkingxe2x80x9d) has been used for centuries in applications such as documents and currency. In current times, the widespread digital representation of images, audio, video and other signals has led to the use of a xe2x80x9cdigital watermarkxe2x80x9d for copyright protection. This marking of digital media involves hiding information under an original signal by embedding (or adding) an imperceptible signal (the mark) to the original signal.
Most marking techniques use existing spread spectrum modulation techniques to modulate information (the mark) and embed the mark within an original signal. For example, the bits composing the desired mark (such as the name of the copyright owner) are modulated using a spread spectrum sequence and added to the original signal (such as a musical work in digital form). More often, the spread spectrum sequence is added to a transformation of the original signal, which may be more sensitive to manipulation. Typically, these existing spread spectrum modulation techniques are robust to interfering noise so that the amount of energy (or distortion) that must be added to the marked signal to erase the mark is quite high. This means that it is difficult remove the mark from the original signal.
In existing spread spectrum techniques, the original signal (also known as a carrier signal) is seen as a source of noise. One problem with these existing techniques is that, as a source of noise, the original signal tends to interfere with the detection of the mark and reduce the mark detection accuracy. This is particularly a problem because the original signal is generally much stronger than any other interference and therefore is the main source of interference.
One embedding technique that uses quantization index modulation does reduce interference from the original signal. The problem, however, with this technique is that the mark is embedded in a lattice, which makes the mark highly sensitive to amplitude scaling of the signal. In other words, a slight change in the scale of the marked signal causes the mark to be erased. This makes the quantization index modulation technique practically useless in applications where a malicious attack may occur.
What is needed is a system and method for embedding information into a digital media that removes the original signal as a source of interference and obtains a large gain accurately detecting the mark within the marked digital media. In particular, the system and method would subtract the influence of the original signal from the mark thereby virtually eliminating a large portion of the interference. Elimination of this interference would greatly decrease error in the mark detection rate and greatly increase performance. In addition, the marking system should be robust and insensitive to amplitude scaling and other forms of malicious attacks. Whatever the merits of the above-mentioned systems and methods, they do not achieve the benefits of the present invention.
To overcome the limitations in the prior art as described above and other limitations that will become apparent upon reading and understanding the present specification, the present invention is embodied in a system and method for embedding digital information into digital or analog media and later detecting the embedded information using a unique spread spectrum modulation technique. The novel spread spectrum modulation technique is capable of removing any carrier signal interference from the modulation. In particular, the present invention subtracts part or all of the interference from the original signal before transmission of a marked signal to enable superior detection of any information embedded in the received signal.
The present invention provides several advantages that improve reliability over current spread spectrum techniques for embedding and detecting information in digital media. In particular, unlike traditional spread spectrum techniques, in the present invention the carrier signal does not act as a noise source because the carrier signal interference is removed beforehand from the received signal. This permits the present invention to achieve significant performance improvements over traditional spread spectrum techniques. Improved performance afforded by the present invention greatly reduces the instances of misdetection of the embedded information. In addition, the present invention maintains the robustness of traditional spread spectrum techniques while achieving higher noise robustness gain and provides security against malicious attacks.
In general, the method of the present invention obtains a received signal on which information is to be embedded. The present invention then obtains information about the expected interference from the original signal upon a watermark detector. The present invention then removes all or part of this interference while simultaneously embedding the desired mark within the signal. Estimation of the amount of watermark signal to embed or remove is performed using a generic function that minimizes the probability of error in the detection of the embedded information. In one embodiment of the invention, the generic function is approximated as a linear function and in an alternate embodiment the generic function is a non-linear function. Moreover, distortion parameters may be introduced in this generic function to control distortion levels. Other enhancements are included within the present invention such as, for example, the introduction of a small randomization into the distortion parameters to provide a more secure detection method.
The present invention also includes a method of reducing the vulnerability of unwanted embedded information removal. In particular, the present invention includes a novel redundant bit representation technique causes a resulting average over a large sample to tend to zero. In a preferred embodiment, the present invention inserts an xe2x80x9cinvertxe2x80x9d bit having a 50% probability to indicate the need to invert any remaining bits. Several other redundant representations may be used with this method to achieve similar results. The present invention also includes a system of embedding and detecting information in digital media that utilizes the methods of the present invention.
Other aspects and advantages of the present invention as well as a more complete understanding thereof will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. Moreover, it is intended that the scope of the invention be limited by the claims and not by the preceding summary or the following detailed description.